Generally, the pump used for drainage of rainwater or the like is often a vertical shaft type pump, with a bearing for supporting the vertical shaft generally being a ceramic bearing having excellent wear resistance. In such a ceramic bearing pump water is used as a lubricant.
Due to rapid urbanization, a problem arises in that a large amount of rainwater abruptly flows into a drainage yard. To cope with this condition, it is necessary to provide for an operation of the pump following a standby operation. When a vertical shaft type pump, which, in a no load operation, pumps air during a stand-by operation, it is driven for a long period of time in such a state, water is not lifted, that is, no lubricating water exists in the pump, during rotation of the main shaft. During the operation, the main shaft rotates in the bearing in a completely dry state, which inevitably results in an abnormal wearing of the bearing. Thus, even a ceramic bearing cannot be employed in the vertical shaft type pump to conduct a no load operation.
As a measure for solving the above-described problem, Japanese Patent Unexamined Publication 2-115592 proposes a method wherein previously prepared lubricating water is supplied from the outside of a pump to a bearing during a previous stand-by operation. In this method, a full-water detector senses whether or not the bearing is soaked within the water and, where the bearing is not soaked within the water, the lubricating water is supplied from outside of the pump, and when the bearing is detected to be soaked in the water, the supply of water is stopped.
Japanese patent Unexamined publication 59-155621 proposes another arrangement wherein, in order to conduct a no-load operation with air for a short period of time, an oilimpregnated ceramic bearing is employed in which oil is previously impregnated and a small amount of oil delivered from the bearing due to thermal expansion lubricates a sliding surface.
Japanese Patent Unexamined Publication 55-90718 proposes a method in which a water storage tank is provided beforehand around a bearing and clean water is supplied into the water storage tank from the outside of the pump, prior to starting operation of the pump.
However, the lubricating structure disclosed in Japanese Patent Unexamined Publication 2-115592 has a disadvantage in that a large amount of water is required for lubrication, when the no-load operation of the pump is driven for a long period of time because the water is continuously fed to the bearing from the outside of the pump during the no-load operation. Also, since the above proposed structure includes a water-supplying pipe system, maintenance of the pipe system is naturally required. On the other hand, only a small amount of oil can be contained in the oil-impregnated ceramic bearing. During the operation of the pump over a long period of time, the temperature of the bearing is unfavorably increased so that it becomes impossible to continue the operation of the pump. Because the oil delivered from the bearing is not recycled, the oil is completely consumed from the bearing during the operation over a long term of years. As a result, the no-load operation may not repeatedly be conducted.
A similar arrangement to the method described in Japanese Patent Unexamined Publication 2-115592 has been proposed wherein a tank or a water storage tank is provided at a bearing portion for the purpose of lubricating the bearing, with the tank including a water-supplying pipe system, so that maintenance of the pipe system is required. When the pumped water is utilized as a lubricant, there is a possibility that sand and soil will deposit in the tank.
In the lubricating structure for the bearing as mentioned above, it is possible to repeatedly and stably conduct the no-load operation for a long period of time while utilizing the water in the pump (pumped water) as the lubricant without supplying lubricating water from outside the pump.